In motor vehicles it may happen that, because of the engine-drag torque, the driven wheels exhibit brake slip in response to a load change. This reduces the cornering stability of the motor vehicle and the motor vehicle becomes unstable. For this reason, it is necessary to reduce the brake slip as quickly as possible. It is conventional to use an engine-drag-torque control for this purpose in order to automatically regulate the engine torque in such a way that the brake torque disappears again as quickly as possible. Nevertheless, in conventional motor vehicles having a gasoline engine, a relatively long time elapses before the increase in the engine torque requested by the engine-drag-torque control has an effect on the driven wheels. The reason for this is that first the opening of the throttle valve and then the air-intake involve a time delay.
In order to prevent a decrease in the cornering stability of the vehicle, so that the vehicle stability remains unaffected, motor vehicles use traction control systems. Such systems, which prevent spinning of the driven wheels and regulate the traction slip to permissible values are described in, for example, “Kraftfahrtechnisches Taschenbuch” [Automotive Handbook], Robert Bosch GmbH, Düsseldorf, 21st edition, pages 555-559. The drive slip is regulated in that, for example, the drive torque is reduced when a limit value of the drive slip is reached. This reduction may be achieved by adjusting the ignition point or the internal combustion engine, or by other conventional methods. Regardless of the manner in which the conventional traction control systems reduce the drive torque once a limit value of the drive slip has been reached, all of the conventional traction control systems have the inherent disadvantage that the traction control system is not only insufficiently controllable, but also the engine adjustment always involves a considerable time factor, so that, overall, a certain time characteristic of the drive torque reduction cannot be achieved.
German Patent Application No. DE 195 49 259 describes a traction control system for hybrid vehicles that avoids the disadvantages inherent in the traction control systems used in conventional motor vehicles. This is achieved by a drive system having a traction control system in which the electromotor is configured in such a way that it reduces the drive slip by lowering the drive torque of the drive unit, this being accomplished by the action of the brake and/or by clutch slip action in electromotors acting as clutch. In this case, the traction control system takes action only when a limiting value of the drive slip is exceeded. If a condition resulting therefrom is then present, this traction control system reduces the drive torque acting on the driven wheels with the aid of the electromotor.